Could the galaxies of the Universe be stalked by rogue, all-devouring black holes? It looks that way. A recent simulation of black holes merger revealed that there could be literally hundreds of rogue black holes scattered across the Milky Way galaxy. Each one would weigh several thousand times the mass of the sun, so if these bad guys exist — why haven’t we identified them already?

“Rogue black holes like this would be very difficult to spot,” says Vanderbilt astronomer Kelly Holley-Bockelmann. “Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby. This produces an effect called gravitational lensing that would make background stars appear to shift and brighten momentarily.”

The research modeled "intermediate mass" black holes. There just one problem; no one even knows if these type of light cannibals even exist. Astronomers do, however, have ample evidence that small black holes less than 100 solar masses are produced when giant stars explode.

They also have evidence that “super-massive” black holes weighing the equivalent of millions to billions of solar masses sit at the heart of many galaxies, including the Milky Way. Theoreticians have predicted that globular clusters –- ancient, gravitationally bound groups of 100,000 to a million stars –- should contain a third class of black holes, referred to as intermediate mass black holes. But so far there have only been a couple controversial observations of these objects. The existence of intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources.

The nature of ultraluminous X-ray (ULX) sources remains controversial, but many astronomers think that some or most of these objects consist of an intermediate-mass black hole that accretes matter from a companion star. Recent evidence suggests that a ULX in the galaxy NGC 5408 shown at top of page has a black hole with about 2,000 solar masses.

In the past few years, scientists have succeeded in numerically simulating black hole mergers that incorporate Einstein’s theory of relativity. One of the most intriguing predictions is that when two black holes that are rotating at different speeds or are different sizes combine, the newly merged black hole receives a big kick due to conservation of momentum, pushing it out in random directions at velocities as high as 4,000 kilometers per second.

“This is much higher than anyone predicted. Even the average kick velocity of 200 kilometers per second is extremely high when compared to the escape velocities of typical astronomical objects,” says Holley-Bockelmann. “We realized that basically any black hole merger would kick the new remnant out of a globular cluster, because the escape velocity is less than 100 kilometers per second.”

Using the facilities of Vanderbilt’s Advanced Center for Computation, Research and Education, Holley-Bockelmann’s team ran a number of simulations of the growth of intermediate mass black holes as they combine with a number of stellar-sized black holes, which are plentiful in globular clusters, paying close attention to the kick they received after each merger.

“We used different assumptions for the initial black hole mass, for the range of stellar black hole masses within a globular cluster, and assumed that the spins and spin orientations were distributed randomly. With our most conservative assumptions, we found that, even if every globular cluster started out with an intermediate-sized black hole, only about 30 percent retain them through the merger epoch. With our least conservative assumptions, less than two percent of the globular clusters should contain intermediate mass black holes today,” she says.

There are about 200 globular clusters in the Milky Way that may have already spawned intermediate-sized black holes, which means that hundreds of them would be wandering invisibly around the Milky Way. These could be engulfing the nebulae, stars and planets that are unfortunate enough to cross their paths, but apparently this poses no imminent danger to Earth – or at least not as far as anyone knows at this point in time.

“These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe,” Holley-Bockelmann stresses. “Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!”

The artist’s concept at the bottom of the page shows an extremely exaggerated-size version of the intermediate-mass black hole that may exist at the center of Omega Centauri. It has the orbital lines of nearby stars drawn in for reference only. Close to the black hole, star motions are faster than those farther away. Such differential velocities are one telltale signature of a black hole’s existence. Most of the clusters stars are cooler stars with a scattering of bluer, hotter stars mixed in.

To deduce the existence of the Omega Centauri black hole, astronomers Eva Noyola (Max-Planck Institute for Extraterrestrial Physics) and Karl Gebhardt (University of Texas, Austin), relied on the combined power of ground-based and orbiting instruments. Using spectra obtained by the Gemini Multi-object Spectrograph (GMOS) at Gemini South in Chile and archive images produced by the Advanced Camera for Surveys on Hubble Space Telescope, they measured the motions and brightness of stars at the heart of this massive cluster in a two-pronged approach that indicated the existence of something very massive hidden among the cluster’s stars.

“Finding a black hole at the heart of Omega Centauri could have profound implications for the past history of the cluster itself,” said Noyola. “Intermediate-mass black holes like this could be the seeds of full-sized supermassive black holes. We may be on the verge of uncovering one possible mechanism for the formation of intermediate-mass black holes.”

Noyola’s observations, made as part of her Ph.D. thesis research under Gebhardt’s direction at the University of Texas, show that there is non-luminous matter at the center of Omega Centauri that is on the order of 40,000 times the mass of the Sun. “If it is a black hole, it’s larger than a stellar black hole but not as large as the supermassive variety,” she said.

Since supermassive black holes are well known to exist in the cores of galaxies, and stellar-mass black holes are found scattered throughout galaxies, astronomers have long sought to find conditions where black holes with masses between these two extremes could form and evolve. “If one was to find a “minuscule galaxy”, that would be a good place to look for an intermediate-size black hole,” said Noyola.

According to Noyola and Gehbhardt, these kinds of black holes could turn out to be “baby” supermassive black holes. “They may be rare and exist only in former dwarf galaxies that were stripped of their outer stars,” said Gebhardt. ”They could also be more common than we expect, existing at the centers of globular clusters as well. If this is true, then they could provide numerous seeds necessary to grow supermassive black holes in the centers of larger galaxies.”